[Bug c++/90159] New: Poor warning for an ambiguous reference

[david.bolvansky at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90159

            Bug ID: 90159
           Summary: Poor warning for an ambiguous reference
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: david.bolvansky at gmail dot com
  Target Milestone: ---

Test case:
#include <queue>
#include <algorithm>

using namespace std;

struct sort_heap
{
        bool operator()(const int* lhs, const int* rhs) const
        {
                return *lhs < *rhs;
        }
};

void foo()
{
    std::priority_queue<int*, std::vector<int*>, sort_heap> tmp;
}

<source>: In function 'void foo()':

<source>:16:59: error: template argument 3 is invalid

   16 |     std::priority_queue<int*, std::vector<int*>, sort_heap> tmp;

      |                                                           ^

<source>:16:61: warning: unused variable 'tmp' [-Wunused-variable]

   16 |     std::priority_queue<int*, std::vector<int*>, sort_heap> tmp;

      |                                                             ^~~


GCC does not provide enough info why argument is really invalid. Clang produces
more info, which explain the root problem:

Clang
<source>:16:50: error: reference to 'sort_heap' is ambiguous

    std::priority_queue<int*, std::vector<int*>, sort_heap> tmp;

                                                 ^

<source>:6:8: note: candidate found by name lookup is 'sort_heap'

struct sort_heap

       ^

/opt/compiler-explorer/gcc-8.3.0/lib/gcc/x86_64-linux-gnu/8.3.0/../../../../include/c++/8.3.0/bits/algorithmfwd.h:576:5:
note: candidate found by name lookup is 'std::sort_heap'

    sort_heap(_RAIter, _RAIter, _Compare);

    ^